Many downhole tools require a pressure buildup in order to be actuated. Some examples of this are liner hangers and various downhole packers. One technique for actuating such downhole tools is to provide a ball seat which captures a ball dropped or pumped from the surface. When the ball lands on the seat, a flowpath is obstructed, thus allowing the surface equipment to build up pressure until a predetermined level is reached. When that level is reached, the downhole tool is actuated. A further pressure buildup in designs previously used would result in breakage of a shear pin or pins. When this occurred, the entire ball seat would be blown out with the ball in place, thus reestablishing a passage in the downhole tubulars. One of the problems with shear pin designs is the inherent lack of accuracy involved in using shear pins. First, inaccuracies in release pressures are inherent to the manufacturing process for the shear pin or pins themselves. Secondly, the manner in which the shear pins are installed, and the materials into which they are installed, sometimes affects the pressure at which these pins will fail in shear. The combination of the variability in their release force required to shear the pins and overcome O-ring friction can be as high as .+-.15% or more in some cases. If the shear pins, for example, are not correctly installed, they do not break cleanly but they tend to extrude, thus affecting the release force required to move the entire ball seat after the downhole tool has set.
Sometimes the shear pins fail prematurely prior to enough pressure having been developed above the ball on the seat to allow the downhole tool to actuate.
It is thus an objective of the present invention to provide a mechanism where the release force required to break loose the ball seat is predictable with more accuracy than has been previously possible to achieve with shear pins alone. Currently, one of the objects of the present invention is to provide an alternative mechanism that can work as the main release threshold level determinant with the possibility of using the device in conjunction with shear pins. Another object is to use the shear pins as temporary fixation devices for the ball seat while using the mechanism of the present invention for accurate and predictable pressure buildup so as to ensure the proper functioning of downhole tools.